Metal oxides, such as ZnO, In2O3, SnO2, ITO, etc., are well known to be sensitive to various ambients and have been employed as electronic gas sensors. The way these sensors work is via charge exchange between a nanowire and adsorbents at the nanowire surface. Due to their inherently high surface to volume ratio, sensors employing nanostructured metal oxides, e.g., nanowires, nanorods, nanobridges, etc., promise ultra high sensitivity. Although metal oxides have been demonstrated to be sensitive to many gases, e.g., O2, H2O, ethanol, methanol, CO, NO2, NH3, etc., one of the great challenges of using these materials is selectivity, or how well a sensor can differentiate the gas of interest from background gases. Kolmakov et al., Chemical sensing and catalysis by one-dimensional metal-oxide nanostructures, Annu. Rev. Mater. Res. 34, 151 (2004).
Currently, several methods have been shown to be at least partially effective in improving sensitivity, such as bias control in a back gated device structure or the use of platinum or palladium nanoparticles to improve sensitivity to H2, as described by Wang et al., Hydrogen-selective sensing at room temperature with ZnO nanorods, Appl. Phys. Lett. 86, 243503 (2005), which discusses careful control of operating temperature, the use of different metal oxide materials in an array, etc. None of these methods have proven to be ideal.
Our prior disclosures, entitled Selective growth of ZnO nanowires using a patterned ALD ZnO seed layer, and U.S. patent application Ser. No. 10/977,430, filed Oct. 29, 2004, and ALD ZnO seed layer for deposition of ZnO nanostructures on a Silicon substrate, U.S. patent application Ser. No. 10/976,594, also filed Oct. 29, 2004, describe processes for fabrication of metal oxide sensors, and are directed towards an integration method for forming nanobridge based ZnO sensors that involves selectively growing ZnO nanowires from one electrode to another, and are incorporated herein by reference.